For long-term drug delivery, ranging from days to months, the parenteral administration of particles in the nano- or micro-scale has been widely used. The nano/micro particles are easy to administer using conventional needles. They have also been used for oral administration of various drugs. Despite extensive applications of the nano/micro particulate systems, the development of clinically useful products has been slow, and only a limited number of clinical products are available. As used herein, “nano” refers to nanoparticles and processes wherein the scale is in the range 1 nanometer (nm) to 1 micrometer (μm). As used herein, “micro” refers to particles and processes in the range of 1 μm to 1000 μm. When it is not necessary to distinguish between nano and micro sizes, the term “micro” is used generically to refer to objects in both the nano- and micro-size ranges.
The slow development of clinically successful formulations is due to several reasons. For example, the drug loading efficiency in microparticles is in general very low; typically below 10% for most drugs, especially protein drugs. Even for those drugs with higher loading efficiency, e.g., 10-20%, the majority of the drug is lost during preparation. This may be acceptable for low cost drugs, but most protein drugs are very expensive and such losses would not be acceptable for any formulations. One of the reasons for such losses is that the water-soluble drugs in the microparticles are exposed to a large amount of water before microparticles become solidified during preparation by emulsion methods, which are the most common method for microparticle preparation. Additionally, current emulsion methods are difficult to scale-up for mass production and result in heterogeneous particle size distributions. Recent advances in nanotechnology, especially in nano/micro fabrication (collectively microfabrication) and manufacturing processes have provided new avenues of making pharmaceutical formulations.
The development of high resolution microfabrication technologies has revolutionized the microelectronics and microdevice industry. By using microfabrication methods, such as photolithography and electron beam (E-beam) lithography, silicon and glass templates with micro and nano features have been developed. In the last few years, several microfabrication methods have been developed as alternatives to E-beam and photolithography that can achieve high resolution without compromising the feature integrity. In addition, these methods possess greater versatility in materials and processing approaches than silicon-based microfabrication techniques. These new printing methods include nanoimprint lithography [H. Schift], step and flash imprint lithography [V. Truskett, et al.], molecular transfer lithography [C. Schaper], and soft lithography [Y. Xia, et al.]. These methods use either microfabricated silicon or glass templates, or polymeric templates with special properties, to form patterns on various substrates.
Nano-imprint lithography (NIL) is a high-resolution patterning method in which a surface pattern of a stamp is replicated into a material by mechanical contact and three dimensional materials displacement. The major advantage of NIL is the ability to pattern sub-25 nm structures over a large area with a high-throughput and low-cost. Step and flash imprint lithography (S-FIL) is distinguished from NIL by being a UV-assisted nanoimprint technique that molds photocurable liquids rather than heat-assisted molding of polymer-coated wafers.
Molecular transfer lithography (M×L) is used for replication of surface patterns as water-soluble templates. The templates are prepared by spin-casting a poly(vinyl alcohol) (PVA) solution on a silicon master pattern. The resultant water-soluble templates are dried and then bonded to another substrate using an intervening adhesive layer that solidifies through photocuring, thermal curing, or two-part reactive schemes. The templates are chemically removed by dissolution with water yielding a formed pattern in the adhesion layer. See, e.g., U.S. Pat. No. 7,125,639 issued to Schaper.
Soft lithography is a collective name for a group of non-photolithographic microfabrication techniques using an elastomeric stamp with relief features to generate microstructures. Among the soft lithographic techniques, microcontact printing [Yan et al.], microtransfer molding [Zhao et al.], and microfluid contact printing [Wang et al.] have produced isolated polymer structures. Soft lithography methods use hydrophobic polymer (e.g., poly(dimethyl siloxane), or PDMS) templates for imprinting. These methods enable preparation of soft material templates using organic polymers, biopolymers, and inorganic materials.
Currently available nano- and micro-particulate drug delivery systems are made mainly by emulsion methods. The emulsion methods result in a highly polydispersed population of particles, and their physico-chemcial characteristics, degradation kinetics, material properties and drug release profiles represent only the average values of the particles. Since the particles are highly heterogeneous, it is difficult to examine the effect of size on biological responses due to wide distribution of the particle size. Furthermore, the presence of particles much larger than the average size makes it difficult to develop clinically useful delivery systems. Microfabrication techniques allow preparation of monodisperse particles. Several soft lithography-based strategies have been developed in combination with a lift-off approach to prepare homogeneous particles. These strategies enable fabrication of microstructures made of drug-containing polymers, even though the processes require substantial improvement for easy collection in large quantities.
MicroContact Hot Printing (μCHP) has been developed to prepare thin-film microparticles with well-defined shapes using thermoplastic polymers [Guan, et al.]. This method selectively transfers polymer features from a continuous film on a stamp to a substrate. By this method, microparticles of thermoplastic polymers, such as poly(lactic-co-glycolic acid) (PLGA), polycaprolactone (PCL), poly(methyl methacrylate) (PMMA), and polystyrene have been prepared [See, e.g., U.S. Pat. Pub. 2004/0115279]. This method suffers from several limitations, however. First, control of the thickness of the microparticles is very difficult as it depends on the complete transfer of the polymer solution from the PDMS stamp to the release slide. This results in formation of microparticles with non-uniform thicknesses. Although the lateral size of the microparticles can be controlled, the vertical size (i.e., the thickness) of the microparticles cannot be exactly controlled. Second, this method is useful only with dilute polymer solutions (1-7%), as it involves filling of the microwells of the template by dipping into a polymer solution. At higher concentrations, the polymer solutions are highly viscous and difficult to fill the wells, as the polymer solutions tend to form a continuous polymer film on the surface of the template. Third, since most of the polymers are soluble only in organic solvents, the solvent can diffuse into the PDMS matrix thus damaging the stamp and preventing smooth release of the microparticles from the wells.
Step and Flash Imprint Lithography (S-FIL) has been used for fabrication of nanoparticles of precise sizes. S-FIL is a commercially available nano-molding process that utilizes the topography of a quartz template to mold UV crosslinkable macromers into patterns on a silicon wafer. [See, U.S. Pat. Pub. 2007/0031505]. Although this method produces nanoparticles, it has serious limitations. First, it involves the in situ photopolymerization of the macromers in the quartz template. This may cause concerns about the purity of the produce particles for clinical applications. Second, some photoinitiator molecules will remain active in the nanoparticles that can react with other biomolecules present in the human body, potentially leading to serious side effects. Third, exposure of the imprinted particles to oxygen plasma during isolation results in the formation of reactive ions on the nanoparticle surface and can also degrade the drug molecules.
Particle replication in nonwetting templates (PRINT) has been developed for making microparticles using fluoropolymer-based templates. Monodisperse polymer particles ranging from sub-200 nm to micron-scale structures of poly(ethylene glycol diacrylate), triacrylate resin, poly(lactic acid) (PLA), and polypyrrole have been fabricated by this method. PRINT uses chemically resistant fluoropolymers as molding materials, which eliminates the formation of a residual interconnecting film between molded objects. [See, e.g., U.S. Pat. Pub. 2007/0264481]. The PRINT mold has been used in fabrication of polymer and protein microparticles [Rolland, et al., Kelly et al.]. The PRINT method has demonstrated the use of non-wetting templates based on fluoro polymers for fabrication of microparticles for various applications. However, practical applications of the PRINT method are limited by the multi-step PFPE template preparation and laborious particle harvesting procedures. For example, the PRINT particles are harvested from the wafer either using physical scraping with surgical blades or by shear force using a glass slide, both of which are not practical and could damage the particles, and thus they may not be suitable for large scale manufacturing. Also, the PRINT particles harvested by using in situ poymerizable cyanoacrylate harvesting films may lead to adsorption of reactive monomers onto the PRINT particles leading to surface contamination.
The methods described above, namely μCHP, S-FIL, and PRINT, are able to produce micro- and nano-particles of homogeneous size and shape, but they have several limitations for practical applications of developing clinically useful drug delivery particles. First, the methods generally require in situ polymerization of the macromers in the template wells as seen in S-FIL and PRINT. This leads to a concern about the purity of the microparticles for human applications. Second, the methods are unable to control the thickness of the microparticles as seen in μCHP. Third, the methods are compatible with only certain materials. Fourth, the methods require multi-step and laborious microparticle harvesting procedures. The rigorous conditions, including highly aggressive solutions and elevated temperatures, which are used to release fabricated microparticles into solution may damage fragile compounds that have been incorporated into the microparticles. Thus, there are significant limitations to using currently available fabrication methods for the preparation of microstructures. In summary, there exists a critical need for development of new template preparation methods and new materials for use in the fabrication of homogeneous microstructures with various sizes and shapes for applications in drug delivery.